This invention relates to precision surface mounted resistors of the general type shown in U.S. Pat. Nos. 3,405,381; 3,517,436; 3,718,883; 4,136,656; 4,172,249 and 4,286,249.
Among the various technologies used to produce resistive electronic components, the foil resistor technology is known to produce very tight resistance tolerances and which have the lowest temperature coefficient of resistance (TCR). Such devices have been made since 1963, as described in detail by the above referenced patents, by bonding of resistive foil to a rigid substrate to form resistive chips. The foil is photo-etched and adjusted to reach the desired ohmic value and tolerance. Copper or gold wire leads or ribbons are attached, mechanically and electrically, to the chip and the assembly is encapsulated in a plastic case or molding in a coating.
With the advent of the surface mounting technology (SMT), molded electronic devices were designed with flat leads protruding from the plastic and formed into xe2x80x9cgull wingsxe2x80x9d, xe2x80x9cJ wingsxe2x80x9d or other forms to facilitate soldering to pads on a printed circuit board (PCB). This technology partially replaced the xe2x80x9cThrough Hole Technologyxe2x80x9d. These assemblies are substantially larger than the chip itself and are expensive to produce compared to production cost of the chip itself.
A reduction in size has been achieved by designing SMT chips with a xe2x80x9cwrap around metallizationxe2x80x9d not necessitating any wiring or molding. These chips have the resistive foil attached to one side of the substrate and are electrically connected to metallization on the other side of the substrate and to it""s lateral surfaces, (see FIG. 1).
It is known in the electronic art, mainly in semiconductors, to produce devices called flip chips or bump devices attached through solder balls to the PCB. However, such devices were never used for Nixe2x80x94Cr alloy precision foil precision resistors. Due to high precision and stability of foil resistors the strain sensitivity becomes a very important factor.
The resistance changes and the resistor are no longer within tolerance when the chip is soldered to the PCB because of strains imposed on the chip during soldering. These changes are in the order of magnitude of 20xc3x9710xe2x88x926 to 200xc3x9710xe2x88x926 cm/cm strain, resulting in resistance changes of about 40 to 400 ppm (parts per million) ohm per ohm. Since most of foil resistors are used with a tolerance of 100 to 1000 ppm (ohm per ohm), this kind of change is not acceptable. Furthermore, in case of a ratio of two resistance values serving for voltage division, the error is further aggravated.
This strain situation occurs mostly due to bending of the chip when the user is mounting the resistor chip onto the PCB. The forces during this mounting and the forces due to further protection and soldering to the PCB produce strains, spoils the initial tolerance achieved during the production of resistors.
Furthermore, after soldering the chip to the PCB an air gap exists between the chip and the PCB impeding heat transfer from the chip to the PCB. Accordingly, the heat transfer is done mainly through the pads. Power handling capability is greatly improved if this heat transfer is improved.
Therefore, the principal object of this invention is to provide a chip resistor with improved characteristics of accuracy, stability, and power handling, and which is less costly to produce.
A further object of this invention is to provide a chip resistor wherein the chip will essentially not be sensitive to bending during and after soldering it to the PCB and the chip being in intimate contact with the PCB (no air gap) will provide a good heat transfer, hence a better power handling of the chip.
These and other objects will be apparent to those skilled in the art.
This invention relates to a resistive Nixe2x80x94Cr foil chip which is directly attached by soldering to the PCB. It occupies essentially only the surface and thickness of the chip itself and enables improvement in accuracy, stability, heat transfer, cost and size when compared to xe2x80x9cwrap aroundxe2x80x9d or molded designs.
The chip includes solder balls or solder tabs and a bending protector plate which is applied to the surface of the chip in a way that after soldering to the PCB the chip is in intimate contact with the PCB, the resistive foil being on the surface close to the PCB. This minimizes the strain due to bending during assembly and soldering onto a PCB. The bending protector can be cemented to the PCB in order to reduce the thermal resistance of the interface. Manufacturing costs are reduced and reduction in space and thickness is achieved. In each case the foil is cemented to the substrate by a cement (shown in FIGS. 1b, 2a and 2b), and protected by a polymer shown only in FIGS. 1b, 2a and 2b.